CFD Analysis of a Canned Pump Rotor Considering an Annular Fluid With Axial Flow

Abstract

Instability in rotor-bearing systems can lead to high levels of vibration, that can ultimately result in the destruction of rotating machines or lost production due to reduced speeds and flow rates. Thus, accurate characterization of instability drivers is vital in prediction of the stability of rotor-bearing-structural systems, allowing for design changes to reduce deleterious effects. These changes include alteration of the system component causing instability and alteration of the bearing design to improve the overall system stability. A source of destabilizing forces in vertical canned motor pumps arises from fluid-structure interaction (FSI) forces between the rotor can and the stator. In this paper, the FSI forces are modeled using an analysis of a long annular fluid filled region with an axial flow component. The model includes the effects of pre-swirl of the fluid entering the annulus. The rotor center static operating position is eccentric to the stator center due to manufacturing tolerances. The fluid forces are calculated using computational fluid dynamics (CFD) and are expressed in terms of equivalent stiffness, damping and fluid inertia (added mass) rotordynamic coefficients. The rotordynamic coefficients are identified using CFD by simulating non-synchronous perturbations of the rotor position and velocity around the static operating point. A separate set of simulations, which does not consider the effects of axial flow on the rotordynamic coefficients, are performed to facilitate direct comparison between the CFD predictions and the dynamic coefficients calculated using the bulk-flow method developed by Fritz.